In response to Stepping Away from OA: A Scientific Conference on the Prevention of Onset, Progression, and Disability of 0steoarthritis by NIH, this proposal seeks to provide new information regarding chondrocyte properties that can be used in the greater effort to determine the role of mechanical factors in OA development and progression (as well as in symptoms and disability). Osteoarthritis (OA) is a painfully debilitating disease, which strikes 5 percent of the general population and 70 percent of Americans over the age of 65 and is responsible for an estimated $28.6 billion per year in related medical costs. Although there has been extensive study of the material properties and behavior of cartilage under biomechanical loading, surprisingly little is known about the inherent properties of chondrocytes, the cells that comprise the living component of cartilage tissue. Many fundamental aspects of cellular function, including shape, deformability, motility, division, viability, and organization of the ECM, appear to be influenced by the mechanical properties of the cell. Therefore, only with a firm grasp of the material behavior of both cells and tissue can the role of biomechanical forces and mechanotransduction events that lead to the development of a normal or pathologic state in cartilage be determined with specificity and underlying cause and-effect relationships be identified. The specific aims of this R21 exploratory grant are: Specific Aim1. The chondrocyte mechanical response to applied loading via osmotic loading will be quantified. Osmotic loading permits the study of cell shape change and deformation without direct application of mechanical force to the cell. Differences in the chondrocyte response during passive swelling (tensile loading) and shrinking (compressive loading) will be characterized using digital video epifluorescence and confocal microscopy as well as atomic force microscopy (AFM). Specific Aim 2. The videomicroscopy data from Specific Aim I will be combined with a triphasic cell model (e.g., solid, fluid and ion phases) to estimate chondrocyte apparent material properties (e.g., aggregate modulus and hydraulic permeability). These properties will be compared with the indentation properties extracted from the triphasic indentation analyses of the AFM data from Specific Aim 1. Successful implementation of the aforementioned will permit new research questions regarding the role of biomechanical influences in the development and progression of OA to be addressed.